In known vehicle speed control systems, typically referred to as cruise control systems, the vehicle speed is maintained on-road once set by the user without further intervention by the user so as to improve the driving experience for the user by reducing workload.
With typical cruise control systems, the user selects a speed at which the vehicle is to be maintained, referred to as a set-speed, and the vehicle is maintained at a target speed that is set equal to the set-speed for as long as the user does not apply a brake or, in the case of a vehicle having a manual transmission, depress a clutch pedal. The cruise control system takes its speed signal from a driveshaft speed sensor or wheel speed sensors. When the brake or the clutch is depressed, the cruise control system is disabled so that the user can override the cruise control system to change the vehicle speed without resistance from the system. If the user depresses the accelerator pedal by a sufficient amount the vehicle speed will increase, but once the user removes his foot from the accelerator pedal the vehicle reverts to the pre-set cruise speed (set-speed) by coasting.
Such systems are usually operable only above a certain speed, typically around 15-20 kph, and are ideal in circumstances in which the vehicle is travelling in steady traffic conditions, and particularly on highways or motorways. In congested traffic conditions, however, where vehicle speed tends to vary widely, cruise control systems are ineffective, and especially where the systems are inoperable because of a minimum speed requirement. A minimum speed requirement is often imposed on cruise control systems so as to reduce the likelihood of low speed collision, for example when parking. Such systems are therefore ineffective in certain driving conditions (e.g. low speed) and are set to be automatically disabled in circumstances in which a user may not consider it to be desirable to do so.
More sophisticated cruise control systems are integrated into the engine management system and may include an adaptive functionality which takes into account the distance to the vehicle in front using a radar-based system. For example, the vehicle may be provided with a forward-looking radar detection system so that the speed and distance of the vehicle in front is detected and a safe following speed and distance is maintained automatically without the need for user input. If the lead vehicle slows down, or another object is detected by the radar detection system, the system sends a signal to the engine or the braking system to slow the vehicle down accordingly, to maintain a safe following distance.
Known cruise control systems also cancel in the event that a wheel slip event is detected requiring intervention by a traction control system (TC system or TCS) or stability control system (SCS). Accordingly, they are not well suited to maintaining vehicle progress when driving in off road conditions where such events may be relatively common. Known TC systems are arranged to intervene and cause application of brake force to reduce wheel speed if the speed of any driven wheel (being a wheel driven by the powertrain, i.e. a driving wheel) exceeds a measured value of vehicle speed, referred to as a vehicle reference speed, by more than a prescribed threshold value (such as 5 kph). The vehicle reference speed may also be used by one or more other vehicle systems that require knowledge of instant vehicle speed.
The present applicant has developed a speed control system suitable for use in off-road driving conditions. The speed control system is configured to allow driving at relatively low speeds, such as speeds in the range from 2 to 30 kph at least, and is configured not to terminate speed control if a wheel slip event is detected requiring intervention by a TC system.
Known speed control systems typically employ a closed loop feedback control arrangement to maintain vehicle reference speed substantially equal to the speed control system target speed. In known cruise control systems, for example, the feedback control arrangement generates a powertrain torque demand signal to cause the value of vehicle reference speed to remain substantially equal to a cruise control target speed.
Methods for determining and providing a value of vehicle reference speed are well known in the art. In some known vehicles the vehicle reference speed value, being an estimated value of vehicle speed over ground, is calculated based on a speed of each wheel, optionally in combination with a measurement of vehicle longitudinal acceleration.
In the case of a two wheel drive vehicle having four wheels, slip of driving wheels, being wheels driven by the powertrain, can be detected as a difference in wheel speed between the driving wheels and wheels that are not driven by the powertrain, on the assumption that the speed of the driving wheels will be greater than that of the non-driving wheels when a slip event occurs. In addition, an expected value of vehicle acceleration for a given amount of wheel torque at the driving wheels may be compared with actual vehicle acceleration as determined by means of an accelerometer in order to identify a situation in which wheel slip is occurring.
In some known vehicles, vehicle reference speed is taken to be the speed of the slowest turning wheel, i.e. the minimum (lowest) measured wheel speed is used as the vehicle reference speed. In some other known vehicles, vehicle reference speed is set equal to the speed of the second slowest turning wheel.
The present applicant has considered the use of such methods of calculating vehicle reference speed and in addition the use of mean driving wheel speed as the reference speed. In particular, in the context of use of such methods of calculating reference speed in a traction control system and an off-road speed control system, when a vehicle is driving in off-road conditions in which surface mu conditions vary.
It is to be understood that if a vehicle has four driving wheels and employs minimum wheel speed as the reference speed for a traction control system, a situation might arise in which up to three of the driving wheels rotate at a speed sufficient to cause the traction control system to intervene to reduce slip, before the traction control system intervenes to reduce slip of those wheels. Slip of three driving wheels on a driving surface may cause not inconsiderable degradation of an off-road driving surface such as mud or grass.
In a four wheel drive vehicle employing mean wheel speed as the vehicle reference speed, no wheel will be controlled to rotate faster than four times the vehicle reference speed. A scenario in which a wheel rotates at up to four times vehicle reference speed under the control of the speed control system may arise for example where a vehicle attempts to accelerate from rest when one wheel is on a driving surface of relatively low surface coefficient of friction (mu) such as wet ice or wet grass whilst the other three wheels are on a driving surface of relatively high mu such as rock or asphalt. It will be appreciated that wheel rotation at up to four times vehicle reference speed may cause substantial degradation of an off-road driving surface.
In the contrary situation where one wheel is on a high-mu surface and three wheels are on a low-mu surface, slip of up to 4/3 of the vehicle reference speed may occur.
Considering a general case of a vehicle having N driving wheels, and in which mean wheel speed is employed by a speed control system to determine actual vehicle speed, the speed control system attempts to make actual vehicle speed equal to the target speed. In the worst case scenario where three wheels are on a surface having a surface coefficient of friction (mu) of substantially one and one wheel is on surface having a surface coefficient of friction of substantially zero, the wheel on the surface where mu is substantially zero may be caused to rotate at a speed of up to N times the target speed, i.e. four times the target speed in a vehicle having four wheels. In a low speed control system, it is conceivable that this speed may be insufficient to trigger a traction control system to intervene to reduce wheel slip. Accordingly, the wheel on the surface where mu is substantially zero may rotate at a speed of up to N times the target speed until a driver intervenes.
In the contrary scenario, in a vehicle having N driving wheels (i.e. N wheels driven by a powertrain), where (N−1) wheels are resting on a surface having a surface coefficient of friction of substantially zero, and an Nth wheel is resting on a surface having a surface coefficient of friction of unity, the (N−1) slipping wheels may be controlled by the speed control system to travel at a speed of N/(N−1) times the target speed. For a four wheel drive vehicle having four road wheels (i.e. a 4×4 vehicle), if the vehicle has three slipping wheels then the slipping wheels may be controlled to rotate at a speed of up to 4/3 the target speed. It is to be understood that, again, in a low speed control system it is conceivable that this speed may be insufficient to trigger a traction control system to intervene to reduce wheel slip. Accordingly, the wheels on the surface where mu is substantially zero may rotate at a speed of up to N/(N−1) times the target speed until a driver intervenes.
It the number of non-slipping wheels increases, then the speed of the remaining slipping wheels may be higher than 4/3 times the target speed.
It is also known to provide a control system for a motor vehicle for controlling one or more vehicle subsystems. U.S. Pat. No. 7,349,776 discloses a vehicle control system comprising a plurality of subsystem controllers including an engine management system, a transmission controller, a steering controller, a brakes controller and a suspension controller. The subsystem controllers are each operable in a plurality of subsystem function or configuration modes. The subsystem controllers are connected to a vehicle mode controller which controls the subsystem controllers to assume a required function mode so as to provide a number of driving modes for the vehicle. Each of the driving modes corresponds to a particular driving condition or set of driving conditions, and in each mode each of the sub-systems is set to the function mode most appropriate to those conditions. Such conditions are linked to types of terrain over which the vehicle may be driven such as grass/gravel/snow, mud and ruts, rock crawl, sand and a highway mode known as ‘special programs off’ (SPO). The vehicle mode controller may be referred to as a Terrain Response (TR) (RTM) System or controller. The driving modes may also be referred to as terrain modes, terrain response modes, or control modes.